1. Field of the Invention
The field of this invention relates to grinding graphite.
2. Prior Art
Graphite is a form of elemental carbon crystallized predominantly in the hexagonal system. Silicate mineral impurities varying in kind and percentage are usually associated with graphite in the ore.
The uses of natural graphite are dependent upon its physical and chemical properties. It is unctuous, which accounts for its demand as a dry lubricant and which, combined with its high electrical conductivity, makes it useful for motor and generator brushes. It is soluble in molten iron and, therefore, is used to raise the carbon content of steel. Its largest single use probably still is for foundry facings, where it prevents metals and alloys from sticking to the molds. Graphite is unequaled for many refractory uses, including crucibles, because of its high heat conductivity, its slow burning property and its ability to retain good strength at high temperatures. It is probably best known, however, for its use in such products as pencils, batteries, paints and inks, and brake linings.
Natural graphite is sold in several different grades. Low grade (low purity) graphite has several substantial uses.
For example, foundry facings have accounted for nearly 20 percent of the total consumption in the past years. Low quality and amorphous graphite is suitable for this use. The graphite is mixed with a small amount of clay, suspended in an adhesive material and applied as thin coatings to mold surfaces to provide for clean and easy recovery of the metal castings.
Another large user of low quality graphite is the steel industry. The graphite is added to steel melts to increase the carbon content to the desired level.
Other uses of graphite require that the graphite have very high purity. High purity natural graphite is required, for example, when the graphite is employed in lubricants or in forming high temperature refractory crucibles. These high purity (premium grade) natural graphites are substantially more valuable than low grade graphite. Flake graphite from disseminated deposits must be concentrated to meet market requirements. Virtually every known concentrating device and combination of separating principles has been tried in an effort to upgrade or purify natural graphite. The mineral has gained a reputation of being difficult to concentrate and upgrade.
Generally, processes for purifying involve an initial step of grinding the graphite to liberate mineral matter. Even this initial step, however, is difficult because the inherent lubricity of graphite makes grinding graphite difficult.
In addition, many uses of graphite require that the graphite be ground to a fine particle size. Graphite lubricants generally require fine size graphite particles. Foundry facings are thin coatings of graphite particles mixed with adhesive material. In pencils and batteries, graphite artifacts are employed which are formed from fine graphite particles and a binder to provide desired shapes and physical characteristics. In short, in many applications, there is a need to grind graphite to provide graphite particles reduced in size.
While a variety of mechanical methods have been employed heretofore for grinding graphite, there is a clear need for a more effective and efficient process for grinding graphite.